This invention relates to the calculation of the minimum spark advance for best torque in a spark ignition-type internal combustion engine.
U.S. Pat. No. 5,887,568 discloses a calculation method for minimum spark advance for best torque ( hereinafter referred to as MBT) in a spark ignition-type internal combustion engine. This method calculates the ignition timing corresponding to MBT from the time period required for the combustion of air-fuel mixture in a cylinder without measuring the pressure inside the cylinder.
Specifically, the total gas weight in cylinder Gcyl and the unburnt gas density in cylinder ROU are respectively calculated, and the ignition timing corresponding to MBT is calculated on the basis of the ratio thereof.
The unburnt gas density in cylinder ROU is a value obtained by dividing the unburnt gas mass by the unburnt gas volume. It is therefore logical to assume that the unburnt gas density can be determined accurately by detecting the values of the unburnt gas mass and unburnt gas volume. In actuality, accurately measuring and estimating the unburnt gas volume in cylinder is difficult. Thus in the aforementioned prior art, the unburnt gas density is calculated on the basis of the cylinder charging efficiency ITAC.
In this case, the charging efficiency ITAC is a mass ratio. However, the volumes of identical masses of gas differ according to pressure and temperature, and it is therefore difficult to calculate the unburnt gas density with precision by means of a calculation which depends only on the charging efficiency ITA.
Moreover, both the total gas weight in cylinder Gcyl and the unburnt gas density in cylinder ROU have units relating to mass. In the prior art, these values are first calculated individually, and division thereof is performed later. However, since the calculation process of Gcyl as well as that of Rou are complicated, the calculation method according to the prior art suffered a huge calculation load.
It is therefore an object of this invention to improve the calculation precision of the minimum spark advance for best torque while achieving a reduction in the calculation load.
In order to achieve the above object, this invention provides an ignition timing control device for use with an internal combustion engine which performs a combustion of an air-fuel mixture in a combustion chamber according to an ignition by a spark plug. The device comprises a sensor which detects an operating condition of the engine and a programmable controller.
The controller is programmed to calculate a reaction probability which expresses a combustibility of the air-fuel mixture in the combustion chamber on the basis of the operating condition, calculate a combustion rate of the air-fuel mixture in the combustion chamber on the basis of the operating condition, calculate an air-fuel mixture combustion period on the basis of a combustion chamber volume at a combustion start timing when the combustion period starts, the reaction probability and the combustion rate to cause the combustion period to be longer as the combustion chamber volume increases, to cause the combustion period to be shorter as the reaction probability increases, and to cause the combustion period to be shorter as the combustion rate increases, calculate a target ignition timing by subtracting the combustion period from a first predetermined timing which is later than the combustion period start timing, and control an ignition timing of the spark plug to coincide with the target ignition timing.
This invention also provides an ignition timing control method for an internal combustion engine which performs a combustion of an air-fuel mixture in a combustion chamber according to an ignition by a spark plug. The method comprises detecting an operating condition of the engine, calculating a reaction probability which expresses a combustibility of the air-fuel mixture in the combustion chamber on the basis of the operating condition, calculating a combustion rate of the air-fuel mixture in the combustion chamber on the basis of the operating condition, calculating an air-fuel mixture combustion period on the basis of a combustion chamber volume at a combustion start timing, the reaction probability and the combustion rate to cause the combustion period to be longer as the combustion chamber volume increases, to cause the combustion period to be shorter as the reaction probability increases, and to cause the combustion period to be shorter as the combustion rate increases, calculating a target ignition timing by subtracting the combustion period from a first predetermined timing which is later than the combustion period start timing, and controlling an ignition timing of the spark plug to coincide with the target ignition timing.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.